Generally, when a compound is used as an active ingredient of a pharmaceutical product, the chemical stability and physical stability of the compound are necessary to ensure stable quality retention and/or easy storage management. Therefore, a stable-form crystal is the preferable final form of the compound. Generally, the most stable-form crystal is often selected as an active pharmaceutical ingredient of a pharmaceutical product. Further, the guidelines for residual solvents of pharmaceutical products in the ICH (International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use) guidelines specify the acceptability and acceptable amount of various solvents. Since solvents to be used for the production of pharmaceutical products are often toxic, in view of safety, it is preferable to reduce residual solvents as much as possible during the manufacturing process.
Currently, a plurality of AKT inhibitors usable as antitumor agents has been reported. Patent Document 1 discloses an imidazooxazine compound (chemical name: trans-3-amino-1-methyl-3-(4-(3-phenyl-5H-imidazo[1,2-c]pyrido[3,4-e][1,3]oxazin-2-yl)phenylcyclobutanol (this compound may hereinafter be referred to as “Compound (1)”)) represented by Formula (1) below, as a compound having a superior AKT inhibitory action and antitumor activity.

However, the crystal form of this compound is completely unknown, and a method for producing a stable-form crystal of high-purity imidazooxazine compound while ensuring sufficient reproducibility has not been reported at present.